1. Field of the Invention
This invention relates to a signal coupling apparatus and a transmitter including a signal coupling apparatus.
2. Description of the Related Art
For example, in a transmitter transmitting high-frequency signals such as a transmitter used in a television broadcasting system, a signal coupling apparatus such as a 3 dB coupler line is used for coupling a plurality of high-frequency signals to amplify them.
JP-A-2002-290101 (KOKAI) discloses a tri-plate type signal coupling apparatus. In the tri-plate type signal coupling apparatus, plates (three dielectric plates and a ground plate) used herein must be brought in close contact with one another. However, the mutual closely contact of these plates in the tri-plate type signal coupling apparatus is easy to loosen. As a result, the tri-plate type signal coupling apparatus can not assure sufficient grounding, so that it is difficult to obtain stable and precise high-frequency characteristic.
As another signal coupling apparatus, a coaxial type signal coupling apparatus is also known. The coaxial type signal coupling apparatus is provided with a coaxial type signal coupling element which includes a slender central electrical conductor having both end parts for providing input and output terminals and an intermediate part between the both end parts, an electrical insulator surrounding the intermediate part of the central conductor concentrically, and an external electrical conductor surrounding an outer peripheral surface of the insulator. The insulator is made of heat-resistant synthetic resin such as polyimide, PTFE (polytetrafluoroethylene), and the like.
After the coaxial type signal coupling element is disposed at a predetermined position on a circuit board, both end parts and a central part of the external conductor in a longitudinal direction of the central conductor are soldered to ground patterns on the circuit board, and the input and output terminals at the both end parts of the central conductor are soldered to predetermined input/output connection patterns on the circuit board.
The coaxial type signal coupling apparatus is further provided with a structure for fixing the coaxial type signal coupling element at the predetermined position on the circuit board securely after the coaxial type signal coupling element is disposed at the predetermined position on the circuit board and electrical connection of the signal coupling element to predetermined circuits on the circuit board is terminated as described above.
This fixing structure includes band members and fixing elements. The band members are made of high heat-conductive material such as copper or aluminum, and these are put on the both end parts and the central part on the outer peripheral surface of the external conductor while the coaxial type signal coupling element is disposed at the predetermined position on the circuit board. The fixing elements, for example screws, are made of high heat-conductive material such as metal and fix both end parts of each band member to the circuit board. Distal end parts of the fixing elements penetrate the circuit board and are connected to a chassis supporting the circuit board and made of high heat-conductive material such as metal.
The band members and the fixing elements fix the coaxial type signal coupling element at the predetermined position on the circuit board securely, and transfer heat generated by the coaxial type signal coupling element while it operates to the chassis made of high heat-conductive material to radiate heat from the chassis.
The thermal expansion of the heat-resistant synthetic resin for the insulator is small, but it is not zero. Accordingly, the insulator expands slightly due to heat generated by the coaxial type signal coupling element while it operates. The thermal expansion of the insulator becomes the maximum in a direction along the longitudinal direction of the central conductor because the insulator has the largest size in the above described direction.
The both end parts of the insulator expanded maximum in the longitudinal direction projects outwardly from the both end parts of the external conductor having heat conductance superior to that of the insulator and having thermal expansion smaller than that of the insulator.
When the operation of the coaxial type signal coupling element is stopped and heat generation by the signal coupling element is also stopped, the insulator contracts. At this time, when both end parts of the outer peripheral surface of the insulator are caught by roughly worked portions on inner peripheral edges of end surfaces of the both end parts of the external conductor being in contact with the both end parts of the outer peripheral surfaces of the insulator, the both end parts of the insulator can not return to their initial positions.
As a result, the both end parts of the insulator cause stresses at the soldered portions on the both end parts of the outer peripheral surface of the external conductor. Such stresses occur at the soldered portions each time when the coaxial type signal coupling element stops its operation, so that durability of the soldered portions is lowered. When cracks or fractures occur at the soldered portions, desired grounding of the coaxial type signal coupling element can not be achieved sufficiently so that performance (namely, stable precise high-frequency characteristic) of the coaxial type signal coupling element lowers.